In order to meet high-integration and high-density trends of semiconductor devices, a photolithography technique for forming fine patterns has been researched and developed. Photolithography methods for improving resolution of fine patterns include a method of adjusting a light source (for example, off-axis illumination), a method using a mask employing optical interference (for example, attenuating phase shift and alternating phase shift), and a method of adjusting the layout of a mask (for example, optical proximity effect).
Since high resolution and depth of focus (DOF) are required to remove an ultrahigh integrated element, a modified illumination method, such as off-axis illumination using an aperture, has been actively developed. Apertures, which are generally used in an off-axis illuminating system, include an annular, a quadruple, and a dipole.
In a semiconductor memory element, such as a DRAM or flash memory in which a plurality of unit cells are repeated, the cells have a designated orientation. Accordingly, in case the above semiconductor memory element is formed by an exposure apparatus employing a dipole as an aperture, it is possible to increase the margin in a process. For example, in case that an exposure apparatus having an off-axis illuminating system employing KrF as a light source and a dipole as an aperture is used, it is possible to achieve fine patterns of a semiconductor element having a half pitch less than 100 nm.
FIG. 1 is a schematic view of a conventional exposure apparatus employing a dipole as an aperture.
As shown in FIG. 1, an exposure apparatus employing a dipole as the aperture, to which an off-axis illuminating system is applied, comprises an aperture 10 serving as means for controlling an exposure light source, a condenser lens 12 for collecting light having passed through the aperture 10, a projection lens 16 for condensing the light onto a wafer 18 placed below the condenser lens 12, and a mask 14 placed between the condenser lens 12 and the projection lens 16 and serving as a reticle for patterning photoresist formed on the wafer 18.
In the above exposure apparatus, after the exposure light is filtered and collected by the aperture 10 and the condenser lens 12 and passes through the mask 14, the exposure light is irradiated onto the wafer 18, provided with the photoresist applied thereon, through the projection lens 16. Thereby, semiconductor device patterns formed in the mask 14 are projected on the wafer 18.
During the above exposing process, a zero-order light component and a negative first-order light component of the light having passed through the condenser lens 12 and the mask 14 are condensed onto the projection lens 16 by a diffraction component, and are irradiated onto the photoresist of the wafer 18, thereby being capable of accurately forming fine patterns of the mask 14.
FIG. 2 illustrates the dipole serving as an aperture. As shown in FIG. 2, the dipole has various pole shapes and orientations, for example, in the vertical direction, in the horizontal direction or at any given angle. Here, the vertical and horizontal directions denote orthogonal directions of geometric patterns, formed along Y and X directions respectively, on a plane.
FIGS. 3A and 3B illustrate wafer patterns exposed by horizontal and vertical patterns respectively having the same size in the mask of the dipole exposure apparatus of FIG. 2 employing the dipole, to which the off-axis illuminating system is applied. A wafer pattern 26, which is made by exposure of a pattern 24 vertical to the dipole, has a resolution higher than that of a wafer pattern 22, which is exposed by a pattern 20 horizontal with the dipole. Accordingly, the horizontal wafer pattern 22 of FIG. 3A has a broad critical dimension, and the vertical wafer pattern 26 of FIG. 3B has a narrow critical dimension.
Differing from FIG. 2, in the case of an illuminating system having poles, through which a laser passes, placed at upper and lower portions thereof is applied to a dipole exposure apparatus as shown in FIG. 4, vertical patterns 32 are laid across a boundary region between an active region 30 and a device isolation region 34 of a semiconductor device formed on a wafer. When the sizes of the vertical patterns 32 are maintained to have small critical dimensions so as to reach the limit of the resolution, the profiles of the vertical patterns 32 are severely deformed at an edge portion (A) of the boundary region, and the critical dimension of the vertical patterns 32 at the edge portion (A) is narrower than the critical dimension of the vertical patterns 32 at a central portion (B).
The above dipole exposure apparatus employing the dipole cannot achieve the fine critical dimension of the vertical patterns, thereby causing the deterioration of yield of semiconductor devices.